In sheet metal stamping operations on materials having reduced ductility, such as advance high strength steels, dual phase steel and aluminum alloys, one issue is that trimmed surfaces tend to split when they are formed into a flange. When a flange is formed on a contoured trimmed surface, the flange is stretched. Current testing methods have proven unreliable in predicting the formability of materials in stretch flanging operations.
In the prior art, a hole expansion test has been used to predict formability. In the hole expansion test, a small diameter hole is formed in a sheet metal blank and then a larger diameter punch is driven through the smaller diameter hole to form flanges. The larger diameter punch is driven through the smaller diameter hole until a through thickness crack appears on the edge of the hole. A limiting hole expansion ratio is calculated based upon the original hole diameter and the expanded hole diameter. Alternatively, a limiting forming ratio may be determined as the ratio of the expanded hole diameter to the original hole diameter.
Tensile testing of a sheared edge may be conducted on sample strips. Samples are trimmed to have a dog bone shape, half dog bone shape or straight sides. The samples are stretched in a direction parallel to the trimming line until failure.
A high degree of correlation is not generally exhibited between the tensile test and the hole expansion test. The expansion test and tensile test methodologies do not provide reliable data on the ability of a particular material specimen to stretch in a specific direction across the width of the flange in a flanging operation. Stretching the flange in the flanging operation may cause significant strain localization in a fracture zone.
Applicant's proposed test methodology has been developed to provide more reliable predictions of metal sample performance in a stretch forming operation that is performed after drawing and trimming operations.